Neuronal circuits in the central nervous system rely on the release of chemical neurotransmitters from specialized connections called synapses for communication. The major excitatory neurotransmitter is the amino acid glutamate, and release of glutamate from a pre-synaptic terminal elicits a response through activation of several types of receptors. One of the sub-types of glutamate receptors, the N-methyl-D-aspartate (NMDA) receptor, plays a major role in neuronal communication and in the plasticity of synaptic responses that occurs under both physiological and pathophysiological conditions.
NMDA receptors are ligand-gated cation channels comprised of a tetrameric assembly of NR1, NR2 and NR3 sub-units (Paoletti and Neyton, 2007). They are unique amongst neurotransmitter receptors in that they require occupation of two separate recognition sites for activation. An acidic amino acid site where glutamate binds, is located on the NR2 sub-units, and a neutral amino acid (or co-agonist) site is located on the NR1 sub-unit. The endogenous co-agonist for this site was originally thought to be glycine, but more recent evidence indicated that D-serine is also an endogenous co-agonist. In fact, in higher brain regions D-serine may be the dominant co-agonist. Occupation of the co-agonist site is essential for glutamate (or a glutamate analog) to activate the NMDA receptor, and in native assays the removal of glycine or D-serine by exogenously-applied degradative enzymes can reduce or abolish NMDA receptor-mediated responses. For example, in the rat hippocampal slice, application of the D-serine metabolizing enzyme, D-amino acid oxidase (D-AAO), completely prevents the induction of long-term potentiation (LTP) a form of synaptic plasticity whose initiation is dependent on NMDA receptor activation (Yang et al., 2003). This suggests that the dominant co-agonist in this case is D-serine, since glycine is not a substrate for D-AAO.
The mechanisms that regulate extracellular D-serine, and therefore govern how NMDA receptors are activated, are not well understood. In keeping with other neurotransmitters and neuromodulators, it is likely that transporters on the cell surface are involved in regulating synaptic levels of D-serine. Amino acid transporters usually prefer L-amino acids, however D-serine has been shown to be a substrate for certain transporters. These include the heterodimeric transporter asc-1 (SLC3A2/SLC7A10) which has micromolar affinity for D-serine, ASCT2 (SLC1A5), ATB0+ (SLC7A9) and PAT1-4 (SLC36A1-4). Based on the tissue and cellular localization, the primary candidates for transporters that regulate synaptic D-serine levels are asc-1 (neuronal) and ASCT2 (glial). The related transporter ASCT1 (SLC1A4) also has been localized to neurons and glia, however it has been reported that D-serine is not a substrate for ASCT1 (Shafqat et al., 1993). None of these transporters are selective for D-serine, and their substrates are typically small neutral amino acids such as serine, alanine, cysteine and threonine. They also are known to function as exchangers that can flux their substrates both into and out of cells. Consequently, it has been unclear if these transporters are responsible primarily for the net uptake or the net release of D-serine and other substrates. Considering that asc-1 has the highest known affinity for D-serine, it has been thought that this transporter is primarily responsible for removing D-serine from the extracellular space (Rutter et al., 2007). In support of this, the asc-1 knock-out mouse has a phenotype that includes increased excitability (Xie et al., 2005).
In the visual system, NMDA receptors are important mediators of glutamate-mediated neurotransmission and synaptic plasticity. This occurs at all levels of the visual axis, including neurons in the retina, in the central neurons that receive retinal ganglion cell input in the lateral geniculate nucleus and the superior colliculus, and in the visual cortex. Based on experiments using D-AAO, D-serine has been shown to be an endogenous co-agonist involved in NMDA-receptor-mediated synaptic responses in the retina (Stevens et al., 2003) NMDA receptors have also been shown to mediate synaptic responses in the lateral geniculate (Harveit & Heggelund, 1990; Scharfman et al., 1990) and the visual cortex (ie the primary pathways that transduce visual information). In the visual cortex, NMDA receptors mediate the phenomenon of long-term potentiation (LTP), an important form of synaptic plasticity. NMDA receptor-dependent LTP occurs in many brain regions and is viewed as a mechanism of synaptic strengthening that is fundamental to the establishment and maintenance of appropriate synaptic connections. In the hippocampus, for example, LTP has been studied as a synaptic surrogate of learning and memory. In visual cortex neurons, LTP mediates stimulus-specific response potentiation, a form of experience-dependent plasticity that contributes to visual function (Cooke and Bear, 2010).
In retinal diseases such as glaucoma and macular degeneration, loss of the vision arises from degeneration or malfunction of retinal cells. Consequently, the normal neuronal transmission along the visual pathway is disrupted in the affected parts of the visual field. One strategy to remedy this loss of function would be to enhance the visual neurotransmission that remains unaffected by disease to compensate for the region of impairment. In addition, enhancing the plasticity of neuronal connections that occurs in the adult visual system could lead to the establishment of new neuronal connections that replace the lost function and improve visual performance.
Enhancing NMDA receptor activity by increasing the extracellular levels of D-serine would boost visual performance and compensate for the loss of vision resulting from retinal disease. As a result, we have discovered compounds that inhibit the transport of D-serine and enhance NMDA receptor-mediated synaptic responses. We identified the D-serine transporters that are important for regulating NMDA receptor-mediated LTP in the visual cortex, and we demonstrated that D-serine transport inhibitors improve visual function in animal models of retinal disease.